It is known in the art that an alkylaromatic hydrocarbon can be catalytically dehydrogenated to form an alkenylaromatic hydrocarbon, such as in the conversion of ethylbenzene to styrene. The prior art teaches a variety of different dehydrogenation catalysts and process parameters, each having different advantages and disadvantages. In general, the prior art teaches that certain tradeoffs ordinarily must be made between level of conversion and level of selectivity, between level of conversion and catalyst life, and so forth. For example, the disadvantage of obtaining a higher degree of dehydrogenation of the alkylaromatic in some processes may be a lower level of selectivity, i.e., a higher percentage of undesired dehydrogenation byproducts. Obviously, it is most advantageous and cost-effective to obtain both high levels of conversion and high levels of selectivity, if possible.
Catalyst life, and the related cost factors, is another important process parameter in these dehydrogenation reactions. First are the costs related to the catalyst itself. Although the unit cost of the catalyst may not be great, because of The large amounts of catalyst required as well as the cost of disposing of used, contaminated catalyst in an environmentally acceptable way, the life of the catalyst and the ability to regenerate used catalyst are critical elements in a commercial dehydrogenation process. Second are the costs related to shutting down a large, perhaps multistage, dehydrogenation reactor, operating at temperatures on the order of 600.degree. C., in order to either replace or regenerate the catalyst bed. In addition to the obvious labor costs, there are also the capital costs of having expensive equipment offline for any length of time. Heat losses add still further costs to this catalyst replacement or regeneration step. Of even greater significance is the cost of lost production during the shutdown period.
Thus, on the one hand, it is preferred to maximize catalyst life. But, on the other hand, normal catalyst degeneration during use tends to reduce the level of conversion, the level of selectivity, or both, resulting in an undesirable loss of process efficiency. Various possible explanations for the typical degeneration of dehydrogenation catalysts during use are found in the literature. These include carbonization of catalyst surfaces, physical breakdown of the interstitial structures of the catalysts, loss of catalization promoters, and others. Depending on the catalyst and the various process parameters, one or more of these mechanisms, or other mechanisms not yet identified, may be at work.
Although the prior art teaches various methods for regenerating used catalyst in order to restore temporarily and only partially the catalyst's effectiveness, these methods generally involve stopping the dehydrogenation, shutting down the dehydrogenation reactor or, in some cases, removing the catalyst for external regeneration. Furthermore, the process impact of such periodic catalyst regeneration is an undesirable saw-tooth pattern of output: levels of conversion and selectivity start out relatively high but slowly and continuously deteriorate until the point where the catalyst is regenerated to restore a relatively high level of conversion and selectivity. But, immediately thereafter, catalyst effectiveness begins again to deteriorate. As a result, it is not possible utilizing conventional catalyst regeneration methods to achieve steady-state process conditions at high levels of conversion and selectivity.
For example, German Patentschrift Nos. DD 298 353, DD 298 354, DD 298 355, DD 298 356, and DD 298 357 teach a 3-step process for regenerating the catalyst bed in an ethylbenzene-to-styrene dehydrogenation comprising: (1) shutting down the reaction and substituting a steam feed for the mixed steam-ethylbenzene feedstream; (2) followed by a heat treatment step; and (3) followed by introducing potassium ions in a steam feed (for example by vaporizing KOH or K.sub.2 CO.sub.3). None of these patents, however, teach or suggest in situ catalyst regeneration without process interruption. The process of these German patents would be costly, cumbersome, and result in the kind of undesirable saw-tooth pattern mentioned above.
U.S. Pat. No. 4,551,571 (Sarumaru et al.) teaches a different approach for extending the life of the catalyst bed in an ethylbenzene-to-styrene dehydrogenation comprising the method of employing two kinds of potassium-containing dehydrogenation catalysts arranged in a particular way within the catalyst bed. U.S. Pat. No. 4,590,324 (Satek) is broadly directed to dehydrogenation of an alkylaromatic compound containing at least two carbon atoms and at least one alkyl group (for example, ethylbenzene) to an alkenylaromatic (such as styrene) by contact with a particular catalyst. Satek's preferred catalyst comprises copper on a support of aluminum borate. At col. 6, lines 57-63, Satek teaches that the catalyst "can be treated or doped with an alkali metal or alkaline earth metal compound for use in the dehydrogenation." Such a one-time doping step is taught as being particularly advantageous for conversion of ethylbenzene to styrene. At col. 6, line 64-col. 7, line 18, Satek also teaches that the oxides, hydroxides and salts of potassium, among others, are suitable agents for doping the catalysts. Satek further teaches that aqueous solutions of the doping agent can be added to feedstocks going to a reactor. However, Satek only suggests doping his particular catalyst (copper on aluminum borate) in the preceding manner. Comparable teachings appear in U.S. Pat. No. 4,645,753 (Zletz et al.), which is referred to as a copending patent application at col. 6, lines 61-63 of Satek.
U.S. Pat. No. 4,902,845 (Kim et al.) teaches a process for extending the life of an iron oxide containing catalyst in alkyl aromatic dehydrogenation processes consisting of adding oxygen or oxygen precursors (such as peroxides) to the reactant feedstream for in situ treatment of the catalyst bed without interruption of the dehydrogenation process. But, the examples and test data presented in Kim et al. demonstrate that this procedure is not really very effective. Indeed, FIG. 1 of Kim et al. suggests that the Kim et al. process at most slows, but does not reverse, the degradation of the dehydrogenation catalyst.
U.S. Pat. Nos. 4,451,686 (DeClippeleir et al.), 5,190,906 (Murakami et al.), 4,064,187 (Soderquist et al.), 4,277,369 (Courty et al.), and 4,287,375 (Moller et al.), are all directed to various embellishments of the conventional ethylbenzene-to-styrene catalytic dehydrogenation process. DeClippeleir et al., for example, suggests one mechanism by which catalyst regeneration might occur in teaching that a small amount of an alkali metal oxide, particularly potassium oxide, "promotes the removal of coke and tars by reaction with steam through the water--gas reaction, and mitigates therefore a carbon build-up on the catalyst surface," (col. 1, lines 39-43). U.S. Pat. No. 4,737,595 (Jones et al.) does not relate specifically to ethylbenzene-to-styrene conversion but does address catalyst regeneration in a method for broadly dehydrogenating dehydrogenatable hydrocarbons. The aforementioned U.S. patents are incorporated herein by reference.
None of the foregoing patents, however, discloses any method for either regenerating or stabilizing catalyst activity in order to maintain substantially steady-state dehydrogenation conditions, over extended periods of time and at very high levels of conversion and selectivity without process interruption. These and other problems with and limitations of the prior art are overcome with the catalyst regenerating and/or stabilizing method and apparatus of this invention.